I know this is possible but can I also do it with my friends computers who are using PC laptops? And is it any different than with Mac laptops. I would have the ehternet plugged into my computer and then what?
It should work with Windows laptops as well as Mac ones. You want to share the internet connection you are receiving over ethernet using Airport? Use the Sharing panel in System Preferences.
What sort of error are they getting? Can they ping the sites from the Terminal (ping http://www.google.com is a good way of testing a connection). If not it's probably that they default gateway and/or DNS setting are not correct.
I use a PC laptop with my Airport. I used to have a mac desktop but that went and died on me. Both worked fine together, I have no problems with the laptop alone. PCs love airports.